Purpose:To evaluate the maco and micro-dosimetric impact of metal inhomogeneities (as seen in patients who have undergone reconstructive surgery) in proton therapy

Methods:Monte Carlo simulations were performed of 3.1mm CP Grade 4 titanium and 3.4mm thick 316 stainless steel plates in a water phantom. Analogous experimental proton dose data was collected in a water phantom using GAFCHROMIC EBT2 radiochromic film (Ashland, Wayne, NJ) and a PTW Markus Ionization Chamber. Beam energy (157 and 225 MeV) and plate placement (eg, at center of modulation) were chosen to be clinically relevant to para spinal and prostate irradiations in the presence of spinal and hip implants, respectively. Film was scanned on an Epson 10000XL scanner and dose maps were calculated from three channel data using FilmQAPro software (Wayne, NJ). Macroscopic dose around the implant was evaluated and impact on proton range was determined and compared to clinical treatment planning system parameters. Dose equivalent and average quality factor distributions around the implant were also studied using Monte Carlo and microdosimetric methods.

Results:Ion chamber data revealed range shifts due to titanium (316 stainless steel) plates of 6.4 to 6.9mm (15.0 to 15.5mm) corresponding to an effective relative density of 3.03 to 3.21 (5.46 to 5.59). Ion chamber data did not show small scale dose redistribution upstream of plates. However, dose data collection and analysis using film (which has higher spatial resolution) is ongoing. In addition, analysis of Monte Carlo simulations will investigate microdosimetric changes and corresponding fluctuations of RBE in plate vicinity.

Conclusion:The presence of high-density materials can introduce changes in the spatial distribution of dose in the vicinity of an implant. This can introduce uncertainty in the dose distribution predicted by proton treatment planning software directly, or due to more subtle consequences of changes in RBE near the material.